User devices (e.g., smartphones, laptop computers, etc.) are often capable of accessing various online services (e.g., email, social networking, banking, shopping, etc.). In order to access such services, a user must often register for the service (e.g., create an account) and provide authentication information (e.g., a username and password) each time the service is accessed. In some scenarios, a user must demonstrate that he or she is an actual person (instead of, for example, a computer) by passing a challenge-response test, such as a Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA) test. A common example of a CAPTCHA test includes showing a user an image that includes a sequence of numbers and requiring the user to enter the sequence of numbers into a textbox. The image may have a skewed perspective, poor lighting, and/or other visual effects to make it difficult for a software program to interpret the numbers in the image accurately.